Scintillators use in high-energy fields of ionizing radiation and in time-resolution apparatus imposes increased requirements to a single crystal in respect to radiation hardness and afterglow in millisecond range. This is of particular importance for cesium iodide scintillator doped by thallium (CsI--Tl) that has attract considerable interest in connection to wide use of photodiodes as scintillation light receivers, since CsI--Tl has a long-wave emission range (.lambda.=550 nm) and, therefore, ensures a high spectral correlation coefficient with photodiodes.
Recently, extensive use is made of gamma spectrometers and electromagnetic calorimeters based on large-size assemblies of modular detectors in form of prisms and truncated pyramids from 250 to 500 mm in height (H). One of main requirements to these elements is, together with high scintillation parameters, the homogeneity of characteristics over the detector height. To this end, large-size scintillation crystals are required characterized by homogeneous radial and axial activator distribution having high radiation hardness. Furthermore, large-size scintillation crystals with homogeneous activator distribution and low afterglow are of considerable interest for detection systems including a large number (several thousands) of small-size scintillators having identical parameters. Among other cases, such a requirement is imposed by the computer tomography.
A scintillation material has been disclosed (U.S. Pat. No. 4,341,654) based on an alkali iodide crystal activated by an optimal amount of effective scintillating dope and containing from 5 to 1000 ppm (of the melt mass) of each complex component of the getter consisting of boron oxides as one component and insoluble but active silicon dioxide as another one.
Method for preparation of this material includes melting of cesium iodide containing raw material, introduction of the activating dope thallium or sodium iodide, as well as additional introduction of boron oxide and silicon dioxide, each in amount of 5 to 1000 ppm by weight, into said raw material, superheating of the melt for time period required to oxides interaction with impurities traces in the melt, with subsequent crystallization and cooling to the room temperature. Authors have pointed out that scintillation materials can be obtained both by Stockbarger and Kyropoulos methods.
Using this method, scintillation materials were grown--sodium iodide doped by thallium (NaI--Tl) and cesium iodide doped by sodium (CsI--Na)--which were colourless, insensitive to one-minute irradiation by a light source with .lambda.=360 nm and having no appreciable afterglow. Yet, there is no information in the method description about CsI--Tl crystals preparation.
A disadvantage of the method mentioned is the settling of the flocks-like interaction products of the getter with the melt. It is particularly a problem when large-size crystals are to be grown since, in this case, the getter amount should be significant enough. Authors state that those floks are displaced into peripheral segments of the ingot. However, a certain probability remains always that those will fall in the crystal bulk. In particular, when crystals are grown by pulling-out from the melt on a seed (Kyropoulos method and modifications thereof), the flocks may emerge to the melt surface where they are captured by the crystal in growth.
Thallium-doped scintillation material on the basis of cesium iodide is known (USSR A.C. N 1,362,088) containing additionally a cesium bromide admixture and having a composition corresponding to the formula EQU (CsI).sub.x (CsBr).sub.y (TlI).sub.1-(x+y)!
where EQU 0.947.ltoreq.x.ltoreq.0.948 and EQU 0.049.ltoreq.y.ltoreq.0.050.
Preparation method of this scintillation material includes melting of a blend containing cesium iodide, activating thallium iodide dope, graphite as a deacidifying agent; additional introduction of cesium bromide in the amount of 5% by mass and thallium in the amount of 0.6 to 1.0% by mass; and subsequent directional crystallization at the residual pressure in the ampoule not exceeding 5 Torr.
Crystals obtained by this method have, in fact, a high transparency to the activator emission (0.005 cm.sup.-1 on .lambda.=560 nm), short decay time (.tau.=0.45 .mu.s) and low afterglow; but authors did not specified scintillation parameters and afterglow for crystals prepared using this method. We have reproduced the conditions of material preparation according to USSR A.C. No 1,362,088 (see Example 1 and Table 1). Disadvantages of that method are its technical difficulty and feasibility in Bridgman-Stockbarger methods only, where graphite comes up into the upper part of ampoule while the melt crystallization starts in the lower (conical) one. When a method of crystal pulling on a seed is used, the crystallization occurs in the surface melt layer where the presence of any suspended particles is inadmissible.
As for quality level of crystals produced by leading worldwide known firms, radiation damage studies of CsI--Tl crystals produced by Quartz et Silice (France) and by Horiba (Japan) have shown that the latter have a rather low radiation hardness, their light yield decrease (.DELTA.C/C) at doses about 10.sup.3 rad being 65%. Crystals of Quartz et Silice are more radiation-resistant, for their .DELTA.C/C at 10.sup.3 rad dose in 15 to 18%, but at 10.sup.4 rad doses, the light yield drops by 30% for large-size articles and less than by 10% for small-size detectors (D. G. Hitlin. G. Eigen "Radiation hardness studies of CsI crystals", Proceedings of the "Crystal 2000" International workship,"Heavy scintillators for scientific and industrial applications" edited by F. Dc Notaristefani et al, "Frontieres", France, 1993, C 58, p. 467-478 ).
Afterglow of CsI--Tl crystals produced by world leading firms is from 0.5 to 5.0% after 3 ms (see, for example. Radiation detectors Catalogue by Quartz et Silice, France, 1990).